The invention relates to a process for electron beam guiding with focusing energy selection in an energy dispersive system with different focusing in two mutually perpendicular directions (especially in energy selection direction and perpendicular thereto in systems focusing in one plane only), and also to electron spectrometers with at least one energy dispersive system with such a beam guidance.
Bundled electrons with given energy are used for the treatment and research of surfaces and gases. For a focusing energy selection, energy dispersive systems are known which are put to use either singly as analyzers or as monochromators or in a combination of an analyzer and a monochromator as a so-called electron impact spectrometer.
Energy dispersive systems as analyzers are used, for example, in UV or X-ray photoelectron spectroscopy (also known by the name ESCA) and in Auger spectroscopy. Here electrons emitted by the specimen are analyzed by the analyzer with respect to their kinetic energy. A lens system situated between the specimen and the analyzer provides for beam transport, the matching of the electron energy to the transmission energy of the analyzer, and also the required enlargement or diminution of the image of the imaged surface of the specimen for matching to the entry slit of the analyzer.
Energy dispersive systems are also used for production of monochromatic electron beams, for example in inverse photoemission spectroscopy. Similarly to the case of the analyzer described above, lens systems are inserted between the monochromator and the specimen for beam transport, and for matching the energy and image size.
In an electron impact spectrometer, the electrons emitted by a cathode are monochromatized in one or more monochromators and focused by a lens system onto a specimen; usually the energy of the electrons at the specimen can be different from the energy in the monochromator. The electrons striking the specimen are scattered by the latter and thereby suffer characteristic energy losses, for example by excitation of vibration quanta. The scattered electrons are conducted by a lens system to the entry slit of one or more energy dispersive elements which analyze the scattered electrons with respect to their energy distribution, and are detected in a detector. Electron spectrometers of this kind are in particular used for vibrational spectroscopy and for investigation of electronic losses on the surfaces of solids, and are made by a number of firms.
In an electron collision spectrometer, the maximum attainable intensity of the beam falling on the specimen, and hence also the intensity of the useful signal produced by the latter, is basically limited by the space charge in the monochromator. Theoretical calculations show (H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982, pp. 16 ff.) that the strength of the monochromatic beam depends on the energy width of the electron beam passed through by the monochromator and can only be influenced to a relatively modest extent by design parameters of the system.
Favorable circumstances as regards space charge result in particular from the use of one or more cylindrical condensers with slits as entry and exit apertures. Focusing of the electrons from the entry aperture to the exit aperture and energy selection here result only in the radial direction, while perpendicular to this neither focusing now energy selection occur. The lack of focusing perpendicular to the radial plane (without the beam guiding according to the invention) has disadvantageous effects on the intensity of the useful signal. The same likewise applies to the analyzer when cylindrical condensers are utilized there.
Attempts have been made (see European Pat. No. 0013003) to compensate for this known disadvantage of cylindrical condensers in that the electrons emitted by the cathode are focused in the radial plane by a suitable lens system onto the entry slit of the monochromator, while perpendicular to this they are focused by a corresponding design of the cathode system and also of the lens system between monochromator and specimen and between specimen and analyzer into an approximately parallel beam path without further intermediate focus on the detector. This beam guiding offers an improvement as against a free non-focusing beam spreading (in the perpendicular direction).
An analogous kind of focusing is described in U.S. Pat. No. 4,559,449.
Closer investigation shows, however, that a series of decisive disadvantages remain. Thus the angle of the beam bundle reaching the detector is small perpendicular to the radial plane, so that according to the fundamentals of optics the intensity remains ;small. Furthermore, perpendicular to the radial plane the beam path at the specimen is nearly parallel.
This means that only a small solid angle of the scattered electrons is caught. Besides this, the described kind of beam guiding is liable to disturbances for small error potentials, which are unavoidable at the frequently used low energies due to inhomogeneities of the surface potentials.